Matteob: As you are a newbie, here's what we should have mentioned first:
Most popular Project Gutenberg books are available from this web site, and often with better formatting. And you don't need a computer for that.
From your Kindle Keyboard browser, go to:
mobileread.mobi
Then select:
Browse Ebooks
and you're on your way. Note that the "PRC" format, like the MOBI format, is for your Kindle.
Getting back to the trouble-shooting arena, I next tried Project Gutenberg Australia (
http://gutenberg.net.au/), and could not get it to work using my Kindle Keyboard alone.
But then I went to Project Gutenberg Canada (
http://gutenberg.ca/), and downloaded, from the "Text" link, a book out of copyright in my country, and -- success. Now, the formatting isn't so good.
The best way to get a Project Gutenberg Canada book onto your Kindle is to download the EPUB version and then convert that to MOBI with the super-popular Calibre (
http://www.calibre-ebook.com/) software widely discussed here. But, on vacation, without a PC, the Canadian "Text" books will work.